Urban heat islands threaten bustling cities by causing dangerously high temperatures, especially in summer. The warming effect comes from the built environment’s ability to absorb and store more heat than natural environments, causing skyrocketing energy consumption and adverse human health impacts.
Construction, architecture and engineering professionals are critical to mitigating this issue. Their building designs and practices allow cities to reduce heat absorption and retention and enhance residents’ safety and comfort.
Understanding Urban Heat Islands
Extreme heat events are proving more detrimental to city dwellers worldwide. In 2022, heat waves across Europe were responsible for about 60,000 deaths, mostly in Spain and Germany. Even more recently, over 100 people became ill — with some hospitalised — after attending a graduation ceremony in Paterson, N.J.
UHIs can raise temperatures by 1 to 7 degrees 17 Celsius, increasing by 15 to 20 degrees Celsius in the afternoon. The built environment is mainly to blame, with asphalt reflecting twice as much heat as grass, tall buildings obstructing cooling winds and looming smog trapping heat at the surface. Waste heat from transportation and a lack of vegetation are also common contributors.
Heat stress in urban communities is dangerous to humans. According to The Lancet, a 1-degree Celsius temperature increase can cause a 2.1 per cent higher chance of cardiovascular disease-related deaths. Heat exhaustion and heat stroke are other possibilities when the body cannot regulate its internal temperature.
Buildings also consume 25 to 50 per cent of energy from heating and cooling. During a heat wave, energy consumption is even higher, releasing carbon dioxide emissions into the atmosphere. Roofs also account for 25 per cent of a city’s total area and receive solar radiation more directly and intensely than any other part of the structure.
Green Building Strategies to Combat Urban Heat Islands
Building and engineering experts pave the way for smart growth by using green materials and construction techniques to tackle UHIs and make cities more resilient.
Green Roofs and Living Walls
At least 70 per cent of roofs in Hamburg, Germany, are vegetation-covered — an initiative that began in 2015 with a €3.5 million investment. After this program achieved desirable outcomes, the city dedicated another €3.5 million in 2023 to continue its green roofing projects on new and existing structures.
Green roofs have proven successful in reducing UHIs. According to the U.S. Environmental Protection Agency, these vegetated surfaces can be 13 degrees Celsius lower than traditional roofs and reduce ambient temperatures by up to 6 degrees Celsius. Building vegetation — including living walls — also affects indoor temperatures, decreasing cooling loads by 70 per cent and indoor air by 2.7 degrees Celsius.
For optimal comfort, indoor humidity levels should be between 40 and 50 per cent, which plants can achieve. These green modifications to the building envelope can enhance occupant comfort and well-being.
Cool Roofs and Reflective Materials
Cool roofs use materials that reflect more sunlight and retain less heat than conventional roofs. They might use reflective coatings, membranes and light-coloured tiles to minimise albedo and heat transfer. New developments in ultra-white coatings are currently underway.
In one study, super-white roofs reduced surface temperatures by up to 11 degrees Celsius in five Chinese cities. This achievement is much more impressive than traditional white roofs, which can reduce indoor air temperatures by a maximum of 2.6 degrees Celsius.
Urban Greening
Planting trees, developing parks and constructing green corridors can significantly reduce the UHI effect in urban areas. Trees shade city blocks and cool the air through evapotranspiration. Meanwhile, green spaces can improve air quality and airflow.
One study in Tacoma, Wash., found that urban trees reduced temperatures by an average of 2.57 degrees Celsius during the summer, depending on plant density. Areas without tree coverage were five times more likely to become dangerously hot during the daytime than shaded areas.
In addition to combating excessive heat, urban greening boosts local biodiversity, promotes public health and creates more liveable, resilient communities for everyone.
Cool Pavements and Permeable Surfaces
In June 2024, 23 people went to the hospital for pavement burns after Las Vegas experienced its hottest June on record. One person passed out from the heat and woke up with burn blisters on 7 per cent of their body.
Cool pavements, like cool roofs, use special coatings and technologies to lower temperatures. Approaches include increasing reflectivity, improving permeability and using moisture within the pavement. The designs minimise heat retention, bouncing back a significant amount of solar radiation and retaining water.
Permeable or porous surfaces store water — usually from rainfall — enabling slow evaporation and cooling as temperatures rise. These solutions make sidewalks and roads less dangerous to the touch.
Future Trends and Considerations
The future of reducing the UHI effect in cities includes additional code adoptions and incentives for green infrastructure. Tax credits and grants for green roofs, cool pavements and urban forestry are also likely. For example, the Inflation Reduction Act already delivers a 30% tax credit for sustainable improvements to commercial and residential structures.
New technologies, including advanced reflective and permeable materials, also have the potential to become the norm in green construction. Architects, engineers and urban planners must integrate UHI solutions early in the planning process as they promote smart growth.
Global cities are already integrating UHI mitigation into infrastructure expansion, adopting updated ordinances and green building codes into their practices. Thirteen states and Washington, D.C., have implemented the International Green Construction Code, which encourages communities to exceed standard requirements for green building standards.
The American Council for an Energy-Efficient Economy also maintains a database of U.S. cities that have spearheaded policies and programs for combating UHIs. For instance, Chicago’s Sustainable Development Policy mandates that new construction projects simultaneously restore natural wetlands. The city also expedites the permitting process for green building projects. In Milwaukee, developers must install green technologies that capture the first half-inch of rainwater.
Building Cooler, Healthier Cities
Cities have little time to waste in mitigating the urban heat island effect, which impacts energy costs and public health. Industry professionals can push cities toward a greener future with solutions like green roofs, urban forests and cool surfaces.